This application claims priority from Japanese Patent Application Nos. 2002-161958 filed Jun. 3, 2002 and 2002-345953 filed Nov. 28, 2002, which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to an optical module used in an optical transmission system and operating at a high speed over a wide band.
2. Description of the Related Art
In recent years, optical transmission systems using optical fibers as a transmission medium have had an increased transmission capacity. Efforts have been made to increase the speed of a circuit such as an optical transmitting and receiving device used in the system and widen a transmission band used by the circuit. For example, a circuit used in a 40-Gb/s optical transmission system is desired to operate over a wide band from about several dozen kHz to about 50 GHz. Among such circuits, MMICs (Microwave Monolithic Integrated Circuits) are known in which an active element, a passive element, and a transmission line are integrated together on a semiconductor substrate to form a circuit providing a plurality of functions. Further, Japanese Patent Application Laid-Open No. 2002-270942 discloses an optical module comprising a housing containing an optical element such as a light emitting element or a light receiving element and an MMIC that forms a driving circuit or an amplifying circuit through integration.
FIG. 1A is a circuit diagram of a conventional MMIC. A power source Vdd is connected to an IC 11. A bias capacitor C1 is connected to the power source Vdd to remove power source noise. The IC 11 is, for example, a pre-amplifier in an optical receiver that operates in a high frequency area. In the MMIC, an LC filter is formed in a power line in order to prevent a high frequency signal in the IC 11 from intruding into the power line as a noise.
FIG. 1B is an assembly diagram of the MMIC shown in FIG. 1A. An inductance element forming the LC filter is generally expensive and has a larger occupied area than the MMIC. Thus, efforts have been made to make a value for an inductance component L1 significant by increasing the length of a bonding wire 14 extending from a pad 13 on an assembly substrate 12 for the power source Vdd to the IC 11.
FIG. 2A shows a configuration of a circuit in a conventional optical transmitter. A driving circuit 52 supplies a bias current to a light emitting element 51 via the inductance component L1. The driving circuit 52 supplies a modulating current corresponding to an electric signal inputted to the light emitting element 61. The value for the inductance component L1 must be increased so as to prevent an AC component of the modulating current from affecting a bias current supplying circuit in the driving circuit 52, the AC component being supplied to the light emitting element 51 by the driving circuit 52.
FIG. 2B shows a configuration of the assembled optical transmitter shown in FIG. 2A. The inductance element is generally expensive and has a larger occupied area than the light emitting element. Thus, efforts have been made to increase the value for the inductance element L1 by increasing the length of a power supplying bonding wire extending from a pad 54 on a heat sink 53 on which the light emitting element is mounted.
However, to obtain the inductance component L1, the bonding wire 14 or 55 must have a length of several cm, requiring an assembly space. Further, owing to its insufficient strength, the wire may be broken. Then, disadvantageously, the circuit may become unreliable. Furthermore, the optical transmitter must have a long bonding wire 56 for a modulating current supply. Thus, disadvantageously, modulation characteristics in a high frequency area may be degraded.
Thus, a method is known which uses a ferrite bead inductor. FIG. 3A shows a configuration of a circuit in an MMIC using a conventional ferrite bead inductor. An LC filter is constructed on an assembly substrate 21 for the power source Vdd by using a bypass capacitor C1 and a ferrite bead inductor L2. FIG. 3B shows an equivalent circuit for a power line. Pads 22 and 23 used to mount parts on the assembly substrate have capacitance components C22 and C23, respectively, if the substrate is made of ceramics and has its back surface grounded. For example, for a ceramic substrate having a relative dielectric constant of 9, a thickness of 200 xcexcm, a pad area of 500xc3x972000 xcexcm, the capacitance components C22 and C23 are each about 0.4 pF.
Resonance occurs between the capacitance components C22 and C23 and the ferrite bead inductor L2. The resonance is reflected in an output from the IC 11 via the power line. Then, disadvantageously, this appears as a gain dip if the IC 11 is an amplifier. This increases jitters in output waveforms.
FIG. 4A shows a configuration of a circuit in an optical transmitter using the conventional ferrite bead inductor. The light emitting element 51 and the driving circuit 52 are connected together via a ferrite bead inductor L11 in place of the bias current supplying bonding wire 55, shown in FIG. 2B. FIG. 4B shows an equivalent circuit for a bias line. Pads 61 and 62 used to mount the ferrite bead inductor L11 have capacitance components C61 and C62, respectively, if the substrate is made of ceramics and has its back surface grounded.
Resonance occurs between the capacitance components C61 and C62 and both the inductance components L63 and L64 of the bonding wires 63 and 64 and the ferrite bead inductor L2.
It is an object of the present invention to provide an optical module that reduces the capacitance components of pads to improve high frequency characteristics.
To achieve this object, an optical module comprises a light emitting element having an electrode to which a bias current is supplied, an interconnect substrate on which an interconnect pattern is formed to supply the bias current, and an inductor part having one terminal connected to the electrode using a connection member and the other terminal connected to the interconnect pattern.
This configuration enables a reduction in the area of a pad to which the inductor part is soldered and reduces the capacitance component of the pad. Accordingly, one of the terminals need not be provided with any pads so that this terminal does not provide the capacitance component of a pad. This improves the high frequency characteristics.
The optical module further comprises a driving circuit connected to the interconnect pattern to supply the bias current to the light emitting element.
The driving circuit includes a bias current supplying circuit that supplies the bias current and a modulating current supplying circuit that supplies a modulating current to the light emitting element. The bias current supplying circuit is connected to the interconnect pattern. The modulating current supplying circuit is connected to the electrode without using any inductor parts.
The connection member may be a bonding wire or a bonding ribbon. Further, one of the terminals of an inductance part may be plated with gold.
In another embodiment, the inductor part includes a first metal block plated with gold and an inductance element having a first terminal to which the first metal block is connected and a second terminal.
The first terminal and the second terminal are plated with at least one of solder, tin, and gold. The inductor part includes a second metal block plated with gold. The second metal block may be connected to the second terminal.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.